Values for the number of neutrons produced per thermal neutron absorbed, ηth, in the three fissile materials U233, U235, and Pu239 are reported. The measurement of these quantities was performed between the years 1951 and 1953 at the Argonne National Laboratory using the original enriched heavy water reactor, CP3', and the technique of pile oscillation. A slight revision of the data has been made on the basis of new total cross sections and f-values. The eta values at thermal energy (0.0253 ev) were found to be η23 = 2.25 ± 0.03, η25 = 2.05 ± 0.03, and η49 = 2.04 ± 0.04.